There is increasing demand for energy in the world today, but reserves of conventional energy sources like fossil fuels are decreasing. Additionally, the use of those natural resources pollutes the environment, all of which leads to increased focus and attention upon renewable energy sources. Gasoline driven motor vehicles are another major consumer of fossil fuel, and consequently people are now trying to replace them with electric motor vehicles or hybrid electric motor vehicles. The problem with electric motors and other machines that require a source of renewable energy, including many consumer products, is the intermittent nature of the power sources, and the need for recharging. Moreover, temporary storage of energy is of utmost importance in renewable-energy-based power plants to level off the load and the supply of energy.
In short, the demand for improved energy storage is great. Consumers want longer-lasting batteries for their portable electronics. Utilities desire more efficient energy storage for grid-scale power leveling. Renewable energy producers are seeking energy storage because many forms of renewable energy are intermittent. People who live in rural areas want energy storage because power is not always available. Hospitals and other critical infrastructure need energy storage to provide for emergencies when power is not available. Further proof of the need for improved energy storage is demonstrated by the fact that tens of millions of dollars are devoted every year by the U.S. government alone, to research and development seeking to improve the lithium ion battery.
Moreover, various documented safety issues with lithium ion batteries have been associated with property damage, injury, and death, including exploding laptop batteries, vehicle fires, and incidents involving airplanes while carrying lithium ion batteries. Lithium ion batteries can fail if they are punctured, or charged too rapidly, or discharged too rapidly, or operated at too high a temperature.
Currently, there is great demand for high density, low weight energy storage technologies for a variety of applications, including portable electronics, electric vehicles, and electric grid stabilization. The most commonly used technology, the battery, does not meet current demand—in spite of their high energy densities, which is typically in the range of approximately 10-100 Wh/kg. Even so, batteries have other limitations, including low power density (approximately 50-200 watts per kilogram, i.e., W/kg), safety issues as previously mentioned, high cost, limited temperature range of operation, low cycle life, incidence of failure, and maintenance requirements which increase the overall cost of a battery.
Rechargeable batteries provide relatively high energy densities, although their power densities are generally lower than larger-scale capacitors. Besides the lithium ion battery, there are various kinds of batteries, which have different energy storage densities. For example, lead-acid batteries: have a relatively low energy density, do not work as well as some other kinds of batteries at low or high temperatures, exhibit a high self-discharge rate, and contain lead, which is being phased out of most applications. Nickel metal hydride batteries provide a higher energy density than lead-acid batteries, and are generally considered safer, but they have a very high self-discharge rate.
Another type of energy storage device is sometimes referred to as a “supercapacitor.” Such a device meets the requirement of high power density (approximately 1000 W/kg), albeit with major limitations including low energy density (approximately 1-10 Watt hour per kilogram, i.e., Wh/kg), high leakage current and high self-discharge rate. There is a need for an improved energy storage device.